Many existing power supplies and electronic ballasts for gas discharge lamps include a DC-to-DC converter circuit, such as a boost converter, a buck converter, or a buck-boost converter, which provides power factor correction and other benefits. DC-to-DC converter circuits generally include at least one controllable power switching device, which is commonly realized by a field-effect transistor (FET).
FIG. 1 describes a common prior art arrangement 10 for a “front-end” portion of a power supply or electronic ballast. Arrangement 10 includes input connections 12,14 for receiving a source of alternating current (AC) voltage 40 (e.g., 480 volts rms at 60 hertz), an electromagnetic interference (EMI) filter 100, a rectifier circuit 200 (which, as commonly realized, provides an output voltage VRECT that is simply a full-wave rectified version of VAC), and a DC-to-DC converter 400 which provides a substantially direct current (DC) output voltage VRAIL that is supplied to a “back-end” portion (not shown in FIG. 1) for powering a load 50 via output connections 26,28.
A significant problem that has been encountered with power supplies and ballasts configured according to FIG. 1 is failure of the FET (within DC-to-DC converter 400) due to momentary overvoltage conditions which occasionally occur in the voltage VAC between input connections 22,24. It should be understood that, in the absence of protective means, an overvoltage condition in VAC directly translates into an overvoltage condition in VRECT.
These momentary overvoltage conditions are generally attributable to the electric utility (e.g., line surges), but have also been observed to occur when AC power is initially applied (i.e., the wall switch is turned on after having been turned off) to the power supply or ballast; it is believed that the latter type of occurrences are attributable to inductive energy stored in AC source 40 and/or in the stray inductance of the electrical wiring that exists between the AC source 40 and input connections 22,24.
The amount of energy in the transients that cause these overvoltage conditions has been found to be, at least in some instances, too large to be effectively suppressed by conventional means, such as a small capacitor, a zener diode, or a transient voltage suppressor (TVS) device. In the absence of effective suppression of these transients, the voltage across the FET exceeds the maximum voltage rating of the device and causes it to fail. The problem is especially acute for higher AC line voltages (e.g., 347 volts or 480 volts) because the peak amplitudes of the transients are correspondingly higher; also, because higher AC line voltages are usually connected to other heavy industrial-type loads, large transients tend to occur more frequently for higher AC line voltages than for lower AC line voltages (e.g., 120 volts or 277 volts).
Thus, a need exists for power supplies and electronic ballasts with a voltage clamping circuit that effectively protects the DC-to-DC converter from damage due to a temporary overvoltage condition at the inputs to the converter. Power supplies and electronic ballasts that include such a voltage clamping circuit would represent a considerable advance over the prior art.